Aristotle was the first to view the development of the chick embryo
and
report the steps of its development. Because all chordate animals
follow
similar patterns of early development, viewing the chick embryo has
much
to teach about development of higher chordates, such as in
humans. It
is especially informative to compare a 40-48 hour embryo with a 70-72
hour
embryo. The earlier show the primative three part brain,
neurulation in progress, fewer somites, the heart is not beating yet,
and the embryo is
still linear. The 72 hour embryo has a 5 part brain, a beating
heart,
many somites, limb buds and it has arched and twisted, all in only 24
hours.

fertile eggs, incubated at 39 C for 72 hrs
(A few at 33 and 48 hrs are instructive, but because the embryos are
smaller, they are more difficult to handle)
39 C Ringer's solution*
in dropper bottle (or other isotonic solution)
Whatman #1 filter paper (or other)

Cut a ring of filter paper, size of a quarter with a
5/8
hole which will encircle the embryo.

2. Pick up an egg from the incubator, keeping
egg in same orientation as in incubator and keep it warm with a
lamp close to
its surface.

With a pencil, mark a circle 3/8th inch bigger
diameter than a quarter on upper surface of egg.

Place the egg in petri dish with paper towel cushion
underneath.
Gently score with a file around the circle with repeated long
slow
strokes. Do not press hard, or egg will break.

(Thanks to Angella Pollitt for for taking pictures in sections
2,
3 and 4.)

3. With scalpel and/or tweezers, flick off the shell
inside the
circle, trying not to break inner shell membrane.

It is not necessary to flick all of the shell off, but you
need
room to cut the shell membrane in the next step.

4. With iris scissors, make shallow cuts to cut inner
shell membrane close
to shell.
The yolk will be floating near the surface with the embryo on
top. Be careful not to pierce the vitelline membrane which surrounds
and contains the yolk, or else you will obscure the operation with
yolk. Flake off additional shell
if more clearance is needed. The embryo should be float in the center
top.

5. Carefully place filter paper circle so that it
encircles the
embryo (still floating in the center of the opened egg, at the top?).
Trim
paper if necessary before putting it in place.

(In this image, the egg has broken into the petri dish, but
the
embryo is still visible.)

6. Carefully cut vitelline membrane just
outside of the filter paper to free the assembly. Do not press down or
the assembly may sink
and be lost in yolk.
(Poke down with one blade, lift
up slightly,
cut the membrane.)

7. When cut free, carefully grasp with tweezers both the
membrane and paper along the edge of the assembly.

Pick up the assembly and gently flush off yolk the
underside
with
Ringers solution.

Transfer to a warm depression slide filled with Ringer's
solution.
Make sure that the embryo does not become detached, nor is not covered
by
the embryonic membranes or filter paper.

Keep it alive by keeping warm and moistened with Ringer's at
all
times.

48 hour embryo:
70 hour embryo:
40hour and 70 hour, same slide

8. Examine under the dissecting scope. Illustrate the
embryo according to the hours of incubation. The beating heart seen in
older embryos will continue for some time providing that it is kept
warm and wet with Ringer's
solution. Draw the embryos in chronological order to show the stages of
development
as seen at hours 48 and 72.